The vehicle was not legally registered, so it was not legal to have on the street. I am not sure that your auto insurer would be able to deny an insurance claim, since you paid the premium on the vehicle, but it may be that your policy of insurance has a clause that it only is effective if the car is legally registered. As for Maryland law, and whether you can be charged as driving uninsured, i would have to look into this more, but if you have proof of paid insurance coverage for this vehicle (and the VIN# of the car and policy match), I would expect to get a prosecutor to drop this charge if i were representing you at trial. Whether the MVA imposes a penalty for having no insurance for all the days you have been driving around, is another question. You do not explain why the driver/owner of the vehicle did not know the vehicle was not legally registered (do not post such facts on this public forum), but that issue will have to be threshed out with your lawyer. You ought to hire one on this.
There is likely coverage. You should take a look at the policy. The declarations page, which lists the vehicles, covered drivers, and amount of coverages, does not list the license plate. Instead, it lists the make model and VIN. You should be fine.